Jill Whalen’s article on “Why Use Keyword Research Tools?", where she answer some of the question put across her. She says —you may not have seen all the features of the subscription keyword research programs such as Wordtracker and/or Keyword Discovery.
You are 100% correct that knowing the top words searched is of no value at all. Some may think they want to optimize for the top words searched, but that would be silly and futile. You can bet that the top 100 words searched each week (heck, probably the top 10,000) are going to be way too competitive to realistically expect to achieve high search engine rankings for them.
You might be wondering what "competitive" means in terms of keywords. In basic terms, the competitiveness of any given keyword or phrase is based on how many pages online there are in the search engines' database which are optimized for the keywords in question.
Many people do a search at Google using their keyword phrase and erroneously judge their competitiveness based on the number of results that are returned. For example, if your phrase is "search engine spam" you might type that into Google (without quotation marks) and see that there are about 40,400,000 results for that phrase.
That's not much help in determining how competitive that phrase is, as it's simply telling you that there are over 40 million pages that Google knows about using the words "search," "engine," and "spam" somewhere on the page, in the indexable code, or in links pointing to that page.